1. Field of the Invention
The present invention related to an apparatus and method for picking up an image of line light applied to the surface of a relatively rotating tire, and for performing shape detection of the tire by the light-section method based on the picked-up image, to thereby detect the surface shape of the tire.
2. Description of the Related Art
The tire has a structure such that various materials such as rubber, chemical fibers, and steel cords are laminated. If there are nonuniform portions in the laminated structure, when air is charged, convex portions referred to as bulges, or concave portions referred to as dents or depressions occur in portions having a relatively low resistance to pressure. Tires causing shape defects such as the aforementioned bulges or dents must be omitted from a shipping object upon being subjected to inspection, from the viewpoint of safety or the viewpoint of the elimination of nonconforming appearance.
Conventionally, the inspection of a tire for shape defects has been made by detecting the surface heights at a plurality of points by means of a contact-type or noncontact-type point measurement sensor while rotating the tire by a rotator, and by detecting the surface shape of the tire based on a distribution of the surface heights. However, in the shape defect inspection based on the shape detection of the tire using the point measurement sensor, the shapes of all faces to be detected for shape defects in the tire cannot be comprehensively detected due to restriction on the number of arranged sensors and restriction on the inspection time. This raises a problem that omission of detection for shape defects is prone to occur.
On the other hand, Japanese Unexamined Patent Application Publication No. 11-138654 discloses a technique for picking up an image of slit light (line light) applied to the surface of a rotating tire, and for performing shape detection of the tire by the light-section method based on the picked-up image, to thereby detect the surface shape of the tire. According to this disclosed technique, it is possible to comprehensively (continuously) detect all faces to be detected (sidewall faces or a tread face of the tire) for shape defects in the tire, thereby allowing the omission of detection for shape defects to be prevented. Here, the term “tread face” refers to a portion of the outer surface of the tire, the portion making contact with a road; and the term “sidewall face” refers to a side portion of the outer surface of the tire, the side portion being located between the tread face and the face of a bead portion, and not making contact with the road.
As shown in the above-described patent document also, in general, when the shape detection is performed by the light-section method, in order that one light section line (a portion to which light is applied, on one line) may be formed on the detection object (sidewall face or the like of the tire), the one line light beam is applied from the detection height direction (direction of the surface height to be detected) in one light section line, and scattered/reflected light beams are captured by a camera arranged in a specific direction to thereby pick up an image of the linear line light.
Meanwhile, the tire surface (especially sidewall faces) is not only of black color but also is highly glossy, and the percentage of the line light applied to the tire surface being scattered/reflected is relatively low. Also, since the tire surface (especially sidewall faces) has a mountain-like shape on the whole, it is necessary to stop down the aperture of a camera in order to obtain a required depth of field.
Therefore, in the surface shape detection disclosed in the foregoing patent document, in order to obtain a clear image of the line light applied to the tire surface, it is necessary to increase the intensity (light quantity) of the line light, or to reduce the image-pickup rate of the camera (i.e., decrease the shutter speed) to thereby increase the exposition time.
However, increasing the intensity of the line light has caused a problem that the tire, which is black and prone to absorb light, may suffer a thermal failure. Furthermore, using a high-power light source (typically, a laser light source) requires a cooling apparatus, thereby resulting in an upsized apparatus, an increased cost, and deteriorated maintenance.
Moreover, when attempting to pick up images of line light beams in the circumferential direction of a rotating tire with a sufficient spatial resolution within a limited time period permitted for a product inspection, undesirably, it is not possible to reduce the image pickup rate (number of image pickups per unit time) of a camera to an image pickup rate low enough to obtain clear images of the line light beams.
For example, the inspection time permitted for a shape defect inspection is about 1 second. Also, in the shape detection of the tire by the light-section method, in order to mutually distinguish the image of line light and the character inscribed on the tire surface from each other, it is necessary to perform an image pickup with a fine spatial resolution of not more than the line width (about 1 mm) of the character. To meet these requirements of the inspection time and the spatial resolution, the image pickup must be performed at 2000 frames per second regarding tires for a passenger car, and 4000 frames per second regarding larger tires for a truck or a bus. However, when performing an image pickup at an image pickup rate as high as 4000 frames per second, the technique disclosed in the foregoing patent document cannot achieve clear images of line light beams.